1. Field of the Invention
This invention relates to electromagnetic interference (EMI) shielding.
2. Description of the Related Art
With much electronic equipment, regulations require shielding to be provided for certain components in order to attenuate the transmission of electrical and magnetic fields which can produce undesired induction currents and static effects. Such shielding is necessarily conductive so that it can be earthed or grounded to prevent a build-up of such currents or static.
Shielding for this purpose has taken the form of covers shaped from an expanded metal sheeting and plastics mouldings coated with metallic paint. It has also been proposed in European Patent Application No. 83401541.4 (Publication No. 100726) to use paper making technology to produce, a web of conductive fibres of up to 6 mm in length and powdered thermoplastics material. The European Patent further proposes the press moulding of a multiplicity of layers of such material into platelets having thicknesses of up to 6 mm which exhibited useful electromagnetic interference shielding properties.
Although European Patent Application No. 83401541.4 (Publication No. 100726) makes reference to the use of metallic fibres of up to 10 mm and of glass fibres of up to 15 mm in length, no example is given of the use of fibres longer than 6 mm or glass fibres longer than 3 mm. This is consistent with the present applicant's experience that it is impossible by conventional wet laying papermaking techniques to produce a satisfactory sheet with fibres longer than about 5 to 6 mm, depending on their thickness. Longer fibres can only be formed into very thin webs from dispersions of very low consistencies (0.1% or less). In forming webs from such dispersions, exceptionally large volumes of water need to be handled in drainage, leading to high capital costs and a very low rate of production. The difficulties in fact make the use of longer fibres impractical using conventional technology.